Self-sufficiency in energy is a stated national goal. Many of the proposed means to achieve this goal, especially those dependent on fossil fuels, are either environmentally unacceptable or not feasible.
Of the available alternatives, solar energy is the most abundant, inexhaustible single resource available. However, capturing and utilizing solar energy is not simple. Methods are being sought to convert solar energy to a concentrated, storable form of energy.
One method of converting solar energy to a usable form being prominently considered is the deployment of large arrays of photovoltaic solar cells, especially in the sunbelt areas such as the southwestern and western regions of the United States. The most promising candidate for the solar cell is a doped silicon material.
Silicon is one of the most plentiful elements in the earth's crust. However, solar cells are presently fabricated from semiconductor-grade silicon, which has a market price of about $65.00 per kilogram. A number of current projects are directed to developing the national capability to produce low-cost, long-life photovoltaic modules at a rate greater than 500 MW per year and at a price of less than $500 per peak kilowatt by the year 1986. A drastic reduction in price of material is necessary to meet these important national objectives. The presence of transition metal impurities has been identified as one of the major factors causing degradation of silicon photovoltaic cells. These impurities have a negative effect on the carrier lifetime and also on the efficiency of silicon cells. Both of these factors have been considered as inter-related critical limitations. The minimization of all transition-metal impurities is a major concern in the production of silicon for wide-spread use in solar arrays.
High quality surfaces of crystalline materials absent defects such as dislocations are required for growing layers which are to be fabricated into electronic devices having optimum performance. Device performance usually depends upon the degree of perfection of the substrate material on which the device is fabricated, because a growing film typically contains all the defects of the substrate from which it grows. This has required the use of expensive substrates of high perfection. Many attempts to produce highly perfect films on imperfect substrates in order to reduce the cost of good devices have been made, but without substantial success.
Growing of films on solid state substrates has been practiced as a means of forming junctions, or for salvaging the substrate. Baliga et al. (U.S. Pat. No. 4,251,299) in FIG. 5 shows non-planar growth of silicon extending beyond etched areas to form a continuous film. However, Baliga et al. use liquid phase epitaxial growth and only want growth to occur in the etched grooves.
There are several references that teach the salvaging of imperfect semiconductor or doped semiconductor blanks. Endler et al. (U.S. Pat. No. 4,255,206) does grow a film extending into and bridging bevels on mesas. However, he only uses a mask to etch the bevels. There is no mask present during liquid or vapor deposition. Mayberry et al. (U.S. Pat. No. 3,559,281) reclaims wafers by stripping the front surface, passivating it and then stripping and polishing the opposite surface and conducting crystal growth thereon.
Barnett et al. (U.S. Pat. No. 3,647,578), Fujii (U.S. Pat. No. 3,671,338), and Revesz et al. (U.S. Pat. No. 3,904,453) do grow crystals on a perforated, masked substrate but they restrict growth to the window openings. Lawrence (U.S. Pat. No. 3,923,567) reclaims crystals by gettering to concentrate defects at the surface. Rode (U.S. Pat. No. 4,050,964) improves smoothness of growth by misorienting the substrate. Lawrence et al. (U.S. Pat. No. 4,062,102) processes reject wafers by stripping and etching to form a pattern for electrodes. MacDonald, Jr. et al. (U.S. Pat. No. 4,131,472) tracks the defects to dies of a mask so as to correct the die openings. Baliga (U.S. Pat. No. 4,128,440) lowers the temperature to control diffusion. Esseluhn (U.S. Pat. No. 4,160,682) shows an improved slider apparatus. Kotval el at. (U.S. Pat. No. 4,124,410) directionally pulls relatively impure silicon to form low cost, single crystal silicon and Mizrah (U.S. Pat. No. 4,199,379) selectively forms a metal pattern on a crystal by deposition, masking and etching.